1. Field of the Invention
The present invention relates to a combiner circuit for signals received via a main antenna and via a diversity antenna of a radio link, utilizing the maximum power method, whereby the phase of one of the signals is varied such that the power of the aggregate signal subsequently formed is optimally high.
2. Description of the Prior Art
In the article of P. D. Karabinis entitled "Maximum Power and Amplitude-Equalizing Algorithms for Phase Control in Space Diversity Combining", published in the B.S.T.J. Vol. 62, No. 1, January, 1983, pp. 63-189, the above-described most standard type of combining main and diversity received signals is described.
This method is beneficial in instances of pure, planar fading (frequency-independent attenuation of the received signals), but is not beneficial in instances having selective fading (frequency-dependent attenuation) of the received signals. In this latter case, combining according to the maximum power principle can, in fact, lead to a powerful, but nonetheless excessively-distorted, received signal. A second combining method is, therefore, recommended, the criterion for the combination phase therein being the minimum frequency response of the aggregate output signal. What is referred to as the minimum dispersion method is described in the article "A Minimum Dispersion Combiner for High-Capacity Digital Microwave Radio" by S. Komaki et al, which was published in the IEEE Trans. Commun., Vol. Com-32, April 1984, pp. 419-428.
This method also has disadvantages in that, in particular, the signal level can drop to such an extent in the attempt to make the frequency response of the aggregate signal low that a receiving outage occurs.
There are, therefore, also proposals to either combine the minimum dispersion method with the maximum power method in that a switch to maximum power is undertaken, given poor signal-to-noise ratios (see European Patent 0 128 812 A2) or in that an evaluation quantity that one attempts to improve by trial-and-error modification of the phase is calculated from the measured receiving parameter. This latter method is known, for example, from the article of Y. S. Yeh and L. J. Greenstein entitled "A New Approach to Space Diversity Combining in Microwave Digital Radio", published in the AT&T Tech. Journal, Vol. 64, No. 4. April 4, 1985, pp. 885-905. In contrast to the maximum power method, the phase must, thereby, be varied by trial and error, i.e., must sometimes also be varied in the wrong direction in order to find the optimum.
In a radio relay system having space diversity reception, the signals received via the main and diversity antennas of the radio link should, insofar as possible, be combined in the receiver such that the bit error ratio (BER) of the demodulated digital signal becomes optimally low and the number of outages (with BER&gt;10.sup.-3) of the received signal is thereby simultaneously minimized. An optimally-effective of the received signals becomes all the more necessary the higher the number of steps in the system (for example, 64 QAM or 256 QAM) and the greater the bandwidth. In the planned employment of two carrier systems, the received signal width is approximately twice as great than in previous signal-carrier systems because of the two sub-bands lying side-by-side. For reasons of expense, however, it is desirable to process both sub-bands in common in a suitable combiner.